drill bits

Cobalt coated HSS drills are for difficult materials, such as stainless steel. Cobalt coatings can withstand higher temperatures than titanium coatings, but are very brittle.

Must admit I've never heard of cobalt coating. I have cobalt drills for my PCB drill, so small sizes only. They are so brittle, you can really only use them in a drill press - they'd break hand held. But stay sharp way longer than HSS, so well worth the extra cost. Cobalt coating presumably allows a degree of drill flexing?

Like Dave, I hadn't heard of cobalt coating for drills but google shows there are all sorts of cobalt coating processes used for a wide variety of applications.

Although Draper products often seem to me to be reasonable quality and good value, this was probably written by an advertising copy writer, rather than a metallurgist.

I had always assumed that the very durable cobalt drill bits (like the Bosch) were in fact a cobalt alloy, perhaps something like 18-4-1 (AISI T1) which is a high speed tool steel dating from the 1950's.

The usual gold coloured coating on quality high speed steel bits is titanium carbide, deposited by CVD (chemical vapour deposition). It has a remarkable effect on tool wear life. Fake coatings are common on cheap bits.

I've stayed away from cobalt, preferring titanium plated. I've read nothing good about cobalt drills.

NT

Then you've read something good here. Very small drills (like say 1mm or so and designed for collet fitting) are very expensive. And the HSS ones go blunt very quickly - and are beyond my skills to sharpen well. Cobalt last extremely well, provided you take care using them.

The coating is very brittle. I would only use them in a machine tool with power feed.

Handy to know.

FWIW very small bits seem to work adequately when sharpened very badly. A symmetrical pointed spade shape seems to be good enough, and is the most simple shape to grind.

NT

Some of the PCBs I make can have hundreds of holes. You really do want a decent drill for this. My PCB pillar drill is an ancient device, but built to withstand Armageddon. And there isn't a centre punched locater or whatever, so you need the drill to start easily with no wandering. A properly sharpened tungsten does all of this with ease.

Oh yes. For several reasons.

Traditional twist drills aren't good performers when it comes to wandering. But its of little import in a drill stand.

NT

It is with a small HSS drill - they can wander quite a bit. Not with rigid tungsten.

I'm sure there are better cutters around - but tungsten twist drills are fine for me at a price I can afford.

On 07/07/2015 15:21, Dave Plowman (News) wrote: ...

That is a job I would normally associate with solid carbide stub drills.

Thanks for all your replies. I notice that a number of them talk about titanium coatings. Coincidentally I recently bought some titanium coated drill bits from Tool station. They sell two types: Silverline and Bosch. I bought the cheaper set; I don't know if that was a mistake?

I have noticed some brands talk about a titanium coating and some talk about titanium nitride. Are they all TiN or are there different types of Titanium coating and are some better than others?

I have noticed some say not to be used on aluminium but I thought that was one of the reasons to coat them in the first place?

I thought they might make a good general purpose set for everything other than masonry. Although if they are that tough, I don't know why I am worried about going through bricks.

Stephen.

NT

IME, the more you pay for any tool, the better it is likely to be, but price is no guarantee of quality.

There is Titanium Nitride (TiN), Titanium Aluminium Nitride (TiAN) and Titanium Carbon Nitride (TiCN). They are all a form of ceramic and increase the resistance of the drill to heat build up, which, in manufacturing, allows you to use higher speeds and thus reduce cycle times. TiN will also have about three times the life of HSS, while the others can give up to five times the life. However, resharpening a coated drill removes the coating and the benefits it gives to the cutting tip.

TiAN will definitely not like being used on aluminium. You need to use TiCN coated, or just bright tip (a polished finish) HSS, preferably with parabolic flutes. Drilling aluminium is also best done with flood lubrication.

For general purpose drills, I would buy simple HSS or cobalt steel jobber drills and ignore the coated types, the advantages of which are mainly of value in manufacturing. For masonry, use masonry drills.

Why not use carbide? Last for ever IME, if you don't bend them.

It is possible to get solid carbide drills which would be good for PCBs although easily broken

I've found that the smallest size of the spade-shaped tile drill very good for starting. Far too big for PCB use, but does a good job of replacing a centre punch as it's so rigid. PITA having to change bit, of course.